The present invention relates to an improvement in the method for the preparation of a double-coated boron nitride article having an undercoating layer of graphite and an overcoating layer of pyrolytic boron nitride exhibiting excellent heat resistance, corrosion resistance and stability against chemicals and useful as a ceramic heater, electrostatic chuck, thermal radiation-shielding board and cylinder or vessel having a heat sink.
Pyrolytic boron nitride, referred to as PBN hereinafter, has various advantageous properties in respects of the excellent heat resistance, corrosion resistance and chemical stability as well as in respect of the high purity to cause no contamination so that PBN is sometimes used as a material of the overcoating layer on a double-coated ceramic article having an undercoating layer of pyrolytic graphite, referred to as PG hereinafter, on a substrate of a ceramic material such as alumina, silicon nitride, silicon carbide, boron nitride, including sintered boron nitride and PBN, and the like so that the excellent properties of PBN are fully utilized to improve the performance of the ceramic article. For example, Japanese Utility Model Kokai No. 5-90880 discloses a double-coated ceramic heater which comprises an integral body prepared by forming an undercoating layer of PG on a substrate body of PBN and forming an overcoating layer of PBN on the undercoating layer of PG. Japanese Patent Kokai No. 5-105557 further discloses a vessel of PBN provided on the inner surface with an undercoating layer of PG to serve as a heat sink or as an electrically conductive layer and with an overcoating layer of PBN on the undercoating layer to serve for electrical insulation and mechanical protection. In such a double-coated ceramic article, it is sometimes the case that, after formation of the graphite undercoating layer, the graphite layer is pattern-wise removed to expose the underlying ceramic surface so that the graphite layer remaining pattern-wise serves, for example, as an electrical heater element in a ceramic heater, the surface of the pattern-wise graphite layer and the exposed ceramic surface as a whole being coated by the overcoating layer of PBN for electrical insulation and mechanical protection.
One of the problems in the above described double-coated ceramic articles having an overcoating layer of PBN is that the overcoating layer of PBN sometimes suffers from occurrence of cracks therein or exfoliation or falling of the overcoating layer from the undercoating layer of, for example, PG to serve as a heat sink. It is therefore eagerly desired to develop an improved method for the preparation of a double-coated boron nitride article having an undercoating layer of graphite and an overcoating layer of PBN exhibiting excellent heat resistance, corrosion resistance and stability against chemicals without suffering from the above described problems and disadvantages in the prior art articles and useful as a ceramic heater, electrostatic chuck, thermal radiation-shielding board and cylinder or vessel having a heat sink.